The present invention relates generally to the field of solar power, and more specifically to power supply circuits used in smart junction boxes (j-boxes) for photovoltaic (PV) solar power modules and related methods of operation.
FIG. 1 is a simplified schematic diagram of a conventional solar power module 10 including: a positive terminal 11 and a negative terminal 12 for connecting the module 10 to a solar array; a plurality of serially connected PV segments 13, each of the segments include a plurality of serially connected PV cells for converting sunlight into electricity; and a conventional j-box 14 that houses bypass diodes 15 (typically Schottky diodes) connected in parallel with each PV segment 13.
For purposes of the present application, there is no official definition of a “smart” j-box, but in the context of this document, a smart j-box is one that contains any kind of active circuitry, rather than just conventional Schottky bypass diodes 15.
FIG. 2 is a simplified schematic diagram of a solar power module 20 with a first type of smart j-box 21. This is the simplest form of a smart j-box 21, wherein the conventional bypass diodes 15 are replaced with active bypass circuits 16. Each active bypass circuit 16 includes: a bypass diode 22; an electronically controlled switch 23; and a control circuit 24 for controlling the switch 23.
When the PV segments 13 are partially shaded, their short-circuit output current (ISC) decreases. If ISC falls below the string current (ISTRING) the bypass diode 22 becomes forward biased. The anode voltage rises above the cathode voltage, causing the control circuit 24 to close the switch 23. When the shade is removed, the polarity of the anode-to-cathode voltage reverses, causing the control circuit 24 to open the switch 23 again.
The key advantage of active bypass is drastically reduced heat dissipation. For example, a typical Schottky bypass diode 15 will have a forward voltage drop of approximately 400 mV at 8 A current, producing 3.2 W of heat in the diode. In contrast, the switch 23 is typically a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) with an on-resistance of about 5 mΩ. Such a MOSFET, conducting the same 8 A current, dissipates just 0.32 W of heat, or 90% less. Reducing the heat dissipation can greatly increase the reliability of the j-box 21, and may even reduce it's cost by eliminating the need for a heat sink.
In a smart j-box such as 21, each bypass diode 22 is typically the body-diode that is an integral part of the associated MOSFET switch 23, rather than being discrete components, like the conventional bypass diodes 15 of FIG. 1.
A smart j-box such as 21 requires a means for powering the control circuits 24. Some prior art examples (e.g., U.S. Pat. Nos. 8,618,864 B2 and 4,869,254 B2) provide this means by making each control circuit 24 a special kind of dc-to-dc converter. When the bypass diode 22 is forward biased, the control circuit 24 converts the relatively small anode-to-cathode voltage into a much larger output voltage that is applied to the gate of the MOSFET switch 23. Once the switch 23 is closed, the anode-to-cathode voltage drops to typically 50 mV. But the special power supply has so much voltage gain (typically at least 100) that it can continue producing enough voltage (typically at least 5V) to keep the associated MOSFET switch 23 fully enhance (completely turned on).
The special power supply circuit in 24 typically produces just a few microamps of output current. While this is perfectly adequate for the task of turning on the MOSFET switch 23, it's simply not enough to do much else.
Yet there is a strong desire in the PV industry for smart j-boxes that can do more than just active bypass. For example, a smart j-box could perform many other useful functions, such as: module-level shutdown (safe mode) for firefighter safety; module-level performance monitoring for improving system efficiency by identifying particular solar modules that are under-performing; arc-fault detection to reduce the risk of fires; and diagnostics (e.g., self-test and arc fault location) to reduce system down-time and maintenance costs.
But these functions require a lot more supply current, typically at least a few milliamps. Providing this much current is not a simple problem because there are at least two basic technical challenges that must be overcome. The first basic challenge is providing power continuously. In the example above, each control circuit 24 only provided power when it's diode 22 was forward biased, and was unpowered the rest of the time. But a smart j-box with all the functions listed above needs power all the time. One reason why providing power continuously is challenging is that the polarity of the voltage between the positive terminal 11 and negative terminal 12 can actually flip; normally the positive terminal 11 is at a higher potential than the negative terminal 12, but when all the bypass diodes are forward biased concurrently the negative terminal 12 is actually at a higher potential than positive terminal 11. Power supply circuits normally require the polarity of their input voltage to be fixed and predetermined.
The second basic challenge is the ultra-wide range of input voltage. When there is no shade on the PV segments 13, the solar module 20 can typically produce up to 36 Vdc between positive terminal 11 and negative terminal 12. But when all the switches 23 are closed concurrently the voltage typically drops below 200 mV. That's a huge ratio of 180:1, and a power supply that can operate over such a wide input voltage range is extremely unusual. The vast majority of power supply circuits operate over an input voltage range of less than 3:1. Furthermore, it is also extremely unusual for a power supply circuit to operate at the ulna-low input voltage of 200 mV.
FIG. 3 is a simplified schematic of a power supply 30 representing a conventional approach that would be obvious to anyone of ordinary skill in the art of power supply design. A battery 31 represents a voltage source that can have either polarity, because it is attached to the circuit via some connector contacts 32a and 32b, which makes it possible to install the battery backwards. Therefore, a polarity correction circuit 33 is included to ensure the input voltage to the dc-to-dc converter 34 is always correct. Typically, the polarity correction 33 is just a plain old bridge rectifier consisting of four diodes, as shown. But in a smart j-box that must operate in safe mode (wherein the input voltage is only about 200 mV), the two diode drops (approximately 1.2V) associated with a bridge rectifier 33 would be unacceptable. So each diode in the bridge would have to be replaced with a MOSFET switch, making a so-called “active bridge”.
This active bridge approach has some obvious disadvantages. First, it requires at least five large MOSFETs (four in the polarity correction 33 and one for the switch inside the converter 34). These would take up a lot of area on an integrated circuit, making it relatively large, and therefore expensive. Second, the challenge of operating at very low input voltage (safe mode) is made more difficult because of the combined on-resistance of these MOSFETs. And third, there would have to be some relatively complex control circuits for opening and closing the four switches in the active bridge.
Therefore, there is a need in the PV solar power industry for a way of powering a smart j-box for PV solar power module that provides power continuously, in all situations including safe mode, and enough power to enable all the advanced functions of a smart PV j-box.